Near-ultraviolet (NUV) measurements of exoplanet transits offer a means to probe atmospheric escape, cloud formation, and planetary magnetic fields. We examine a 2024 XMM-Newton Optical Monitor NUV observation of the transit of XO-3 b, a massive hot Jupiter on an eccentric orbit with a previously observed abnormally large NUV-absorbing atmosphere. We analyze these NUV data jointly with a concurrent ground-based optical observation and all Transiting Exoplanet Survey Satellite transit observations, and find a NUV transit depth of Rp,NUV/R⋆=0.1371−0.019+0.016 , which is 30%─70% deeper than the optical transit. Although the optical transits do not show signs of transit timing variations, the transit center in the NUV is 22−11+13 minutes late relative to the optical ephemeris. We investigate atmospheric escape as a potential explanation for the properties of this NUV transit by examining X-ray data from XMM-Newton, characterizing the X-ray luminosity of XO-3 for the first time and estimating an extremely small mass-loss rate of ∼104 g s−1 (∼10−19 Mjup yr−1). Finally, we investigate the likelihood of an NUV-absorbing bow shock by estimating the magnetic field of the planet. While such a mechanism can in principle produce NUV transit offsets on the order of tens of minutes, our analytic approximations predict an early rather than late transit, indicating the need for further magnetohydrodynamic simulations.